This invention relates to a DC to AC inverter and, more particularly, to such an inverter in which a multiple step approximation to a sinusoidal wave is developed by summing in phase rectangular waves of different frequency.
Inverters are used in a wide variety of applications where only DC power is available, but AC power is needed. Inverters are also used in situations where AC power is available, such as aboard an aircraft, but is unregulated with respect to frequency. In such situations, the available AC power from generators or the like is converted to DC power, regulated with respect to amplitude, and then reconverted to AC power by an inverter which establishes a fixed frequency therefor. In most of these applications it is desired to minimize the number of harmonics in the AC output signal, and in airplane application, in particular, it is most important that the weight of electromagnetic components of the inverter be minimized.
Known DC to AC inverters have used a variety of techniques to synthesize a sinusoidal voltage waveform. In U.S. Pat. No. 3,781,635 of Sauer, a three-phase inverter is shown in which each of the three stages has a voltage divider coil that is appropriately switched to negative and positive sources of voltage to produce a stepped waveform with three steps in each half wave. Because of the phase displacement between the three output waveforms, three phase-to-phase output waveforms approximating sine waves are produced, each of which has three incremental steps in each quarter cycle.
Another three-phase inverter is shown in U.S. Pat. No. 3,573,602 of Jensen in which each composite output wave is formed by adding to a basic impulse occupying substantially the entire amplitude-time area of each half cycle, a pair of symmetrically disposed impulses of substantially less amplitude than the main impulse. The pair of symmetrically disposed impulses are intended to eliminate harmonics, and each is produced by a pair of windings connected in series between an output terminal and a center tap of a choke of the stage associated therewith which are inductively coupled with the chokes of the two other stages, respectively.
In U.S. Pat. No. 3,638,094 of Ve Nard, II, a three-phase inverter is disclosed in which a composite waveform, shown in FIG. 7C, is developed by adding a basic impulse similar to that of Jensen to a stepped waveform similar to that of Sauer and having the same frequency as the basic impulse.